As semiconductor manufacturing advances to ultra-large scale integration (ULSI), the devices on semiconductor wafers shrink to sub-micron dimension and the circuit density increases to several million transistors per die. In order to accomplish this high device packing density, smaller and smaller feature sizes are required. This may include the width and spacing of interconnecting lines and the surface geometry such as corners and edges, of various features.
The requirement of small feature sizes with close spacing between adjacent features requires high resolution photolithographic processes. In general, photolithography utilizes a beam of light, such as U.V. waves, to transfer a pattern from a photolithographic mask onto a photoresist coating through an imaging lens. The mask includes opaque and transparent regions such that the shapes match those of the openings in the resist coating in the desired or predetermined pattern.
One technique currently being investigated for improving the resolution of the photolithographic process is known as phase shift lithography. With phase shift lithography the interference of light rays is used to overcome diffraction and improve the resolution and depth of optical images projected onto a target. In phase shift lithography, the phase of an exposure light at the object is controlled such that adjacent bright areas are formed preferably 180.degree. out of phase with one another. Dark regions are thus produced between the bright areas by destructive interference even when diffraction would otherwise cause these areas to be lit. This technique improves total resolution at the object and allows resolutions as fine as 0.15 .mu.m to occur.
An early patent in this field, U.S. Pat. No. 4,360,586 to Flanders et al, was issued on Nov. 23, 1982 and assigned to MIT. This patent was directed to exposing periodic optical features on an object surface. The features were characterized by a spatial period p. According to the invention, a source of radiant energy of wavelength .lambda. illuminates a surface to be exposed through a mask having a spatial period separated from the surface by a distance approximately S.sub.n =p.sup.2 /n.lambda., where n is an integer greater than one.
With respect to semiconductor fabrication numerous laboratory techniques have been proposed to employ phase shifting in the photopatterning of semiconductor wafers. Most of the work in this area has centered around either "Alternating Phase Shifting", "Subresolution Phase Shifting", or "Rim Phase Shifting" experiments. In general, in each of these techniques a phase shift mask or reticle is constructed in repetitive patterns of three distinct layers of material. An opaque layer on the mask provides areas that allow no light transmission therethrough, a transparent layer provides areas which allow close to 100% of light to pass through and a phase shifter layer provides areas which allow close to 100% of light to pass through but phase shifted 180.degree. from the light passing through the transparent areas. The transparent areas and phase shifting areas are situated such that light rays diffracted through each area are canceled out in a darkened area therebetween. This creates the pattern of dark and bright areas which can be used to clearly delineate features of a pattern on a photopatterned wafer.
"Alternating Phase Shifting" as disclosed in [1] is a spatial frequency reduction concept similar to the method disclosed in the Flanders et al patent. It is characterized by a pattern of features alternately covered by a phase shifting layer. "Subresolution Phase Shifting" [2] promotes edge intensity cut off by placing a subresolution feature adjacent to a primary image and covering it with a phase shifting layer. "Rim Phase Shifting" [3] overhangs a phase shifter over a chrome mask pattern.
In general, these phase shifting techniques have not been adapted to large scale semiconductor manufacturing processes. One problem with applying phase shifting lithography into practical use, in manufacturing semiconductors, is the difficulty in reticle mask making, inspection, and repair. The process must be compatible with manufacturing conditions, (i.e. inexpensive, repetitive, clean) and prior art laboratory techniques have not heretofore met these criteria.
A representative state of the art semiconductor laboratory process for making a rim phase shift mask or reticle is shown in FIGS. 1A-1C. A transparent quartz substrate 10 has a film of an opaque light blocking material 12 such as chromium (CR) deposited thereon. A transparent phase shifter material 14 such as (SiO.sub.2) is then deposited onto the opaque film 12. The phase shifter material 14 is selected with an index of refraction and with a thickness "t" that preferably produces a 180.degree. (.pi.) phase shift for light passing therethrough. This thickness "t" can thus be determined by the formula: ##EQU1## where
t=thickness of phase shift material
i=an odd integer
.lambda.=wavelength of exposure light
n=refractive index of phase shifter material at the exposure wavelength
Next, and as shown in FIG. 1B, a layer of resist 16 is deposited and patterned onto the phase shifter layer 14. After etching and stripping steps, this forms a pattern of opaque light blockers 18 each having a rim phase shifter 20 formed thereon.
Next, and as shown in FIG. 1C, an isotropic wet etch is performed to undercut the opaque light blockers 18 to form wings 22 of phase shifting areas along the rim of the pattern created by the opaque light blockers 28. The finished reticle 24 can be then used as a mask as shown in FIG. 2A in photopatterning a wafer.
As illustrated graphically in FIGS. 2B-2C phase shifting only takes place on opposite sides (i.e. the rim) of the patterns formed by the opaque light blockers 18. The areas of zero light intensity correspond to the pattern made by the opaque light blockers 18. This prevents large areas of negative amplitude that promote an undesirable intensity along the dark edges of the image. As is apparent, the only image improving function of rim phase shifters is in edge contrast enhancement.
There are several inherent problems with such a method of producing rim phase shifters 20. Firstly, undercutting the opaque light blockers 18 may cause rough edges to be formed on the opaque light blockers 18. This edge roughness may be transferred to the printed wafer
Secondly, the overhanging wings 22 of the rim phase shifters 20 can flake off during cleaning and handling of the reticle. This may cause contaminants in the process and produce patterning errors in the finished wafer.
Moreover, a reticle formed by such a process may be very difficult to repair. If a wing 22 of a rim phase shifter 20 breaks off, for instance, it cannot easily be replaced. Moreover, the distance of the wing 22 overhang is difficult to accurately control.
In addition, the thickness of the rim phase shifters 20 must be accurately and uniformly formed. Any variation in the thickness of the rim phase shifters 20 will adversely affect the phase shifted image. With a deposited phase shift material, it may be difficult to accurately control this thickness.
The process of the invention is directed to a process that overcomes these prior art limitations and provides a clean, repetitive, technique for forming accurate rim phase shifting reticles suitable for large scale semiconductor manufacturing.